The present invention relates to a dot line printer, and more particularly, to a type thereof having an improved comb yoke.
A dot line printer generally provides a hammer bank which secures spring charged printing hammers arranged in side by side in a shuttling direction. During reciprocal movements of the hammer bank, the print hammers are selectively operated to provide a dot impression image on a printing sheet. Incidentally, throughout the specification, the term "shuttling direction" is used to indicate a transverse direction of a printing sheet or reciprocating or shuttling direction of a hammer bank which includes a printing hammer assembly, and the term "line to line direction" indicates a feeding direction of the printing sheet.
One example of a conventional dot line printer is shown in FIG. 1. The printer includes a hammer bank 3 which secures printing hammers (not shown in FIG. 1). The hammer bank 3 is reciprocally movable by a shuttle motor 1 through a cam member 2 in the shuttling direction X. Upon a single rotation of the shuttle motor 1, one reciprocation of the hammer bank 1 is provided. The cam 2 is coupled to a cam shaft to which an encoder 4 is connected. The encoder 4 is formed with a plurality of slits indicative of a reciprocating position of the hammer bank 3. Further, a sensor 5 is positioned in a vicinity of the encoder 4 so as to detect the slit. A platen 6 extends in the shuttling direction X, and an endless ink ribbon 7 also extends in the shuttling direction at a position between the hammer bank 3 and the platen 6. Furthermore, a sheet feed motor 10 is provided, and a pin tractor 9 drivingly connected to the sheet feed motor 10 is also provided for feeding a printing sheet 8 in the line to line direction as indicated by an arrow Y. The printing sheet 8 is adapted to pass through a space defined between the ink ribbon 7 and the platen 6. Upon selective actuation of the printing hammers, the hammers are moved toward the platen 6, so that the intermediary ink ribbon is selectively depressed by the printing hammers to provide an ink image on the printing sheet 8.
FIG. 2 shows a conventional spring charged type hammer bank assembly 3. The assembly includes a hammer base 14 extending in the shuttling direction and having a front face, and a plurality of leaf spring type printing hammers arranged side by side in the shuttling direction. The printing hammers comprise hammer springs 11. The hammer springs 11 have free ends provided with printing pins 12, intermediate portions provided with plungers 13 formed of magnetic material, and a base end portion. Further, a front yoke 15 is provided in front of the hammer springs 11, and the base end of the hammer springs 11 and the front yoke 15 are fixed to the front face of the hammer base 14 by screws 16. A comb yoke 18' (FIG. 3) also extends in the shuttling direction and at a position behind the hammer springs 11, and a front portion of the comb yoke 18' defines pole portions 18a (FIG. 3). A permanent magnet 17 is interposed between the comb yoke 18' and the hammer base 14 for allowing the free end portion of the hammer springs 11 to be attracted to the pole portions 18a in order to provide non printing position of the hammer springs 11. Further, electromagnetic coils 19 are wound over the pole portions for selectively releasing the free end portions of the hammer springs 11 from the associating pole portions and for directing the free end portions toward the printing sheet in order to perform dot line printing.
In order to perform high speed dot line printing, the printing hammers must be arranged at high density. However, with the above described spring-charged type printing hammers, several deficiencies may result for the high speed dot line printing. More specifically, as shown in FIG. 3, the comb yoke 18' has a plurality of pole portions 18a and a base portion 18b joining together the pole portions. The pole and the base portions are integrally provided and the integral comb yoke 18' is formed of a magnetic material such as silicon steel. The numbers of the pole portions is equal to or more than the numbers of the printing hammers, and the pole portions are arranged side by side at a constant pitch P1 corresponding to an array of the printing hammers.
The pitch P1 of the pole portions 18a must be as small as possible in order to provide the high density arrangement of the hammers. Here, in order to attract the printing hammer to the pole portion, sufficient amount of magnetic flux must be required. Therefore, a material capable of providing highly saturated magnetic flux density must be used as a material of the comb yoke. In this connection, Permendur has been used as the material. Permendur is a magnetic alloy which is composed of equal parts of iron and cobalt and has an extremely high permeability when saturated. However, Permendur is an extremely expensive material, e.g., ten times as expensive as silicon steel. Accordingly, the resultant comb yoke becomes expensive.
Further, in a high density arrangement of the printing hammers for the purpose of high speed printing, heat generation amount per unit area becomes large at the printing hammer portion. Accordingly, insufficient cooling to the hammer bank results. Moreover, large electrical power consumption results in the high speed printing.